The present invention relates to a gas mixture separator utilizing pressure modulation which is used, for example, in a fuel power plant and uses adsorbent that has pressure dependent adsorptivity, such as zeolite.
The typical prior examples of gas mixture separators utilizing pressure modulation (referred to as PSA hereafter) are disclosed in Japanese Patent Publication No. 20082/1970, Japanese Patent Provisional Publication No. 14070/1972, and Japanese Patent Provisional Publication No. 16874/1973. In any of the above prior art references, gases are separated from a mixture by: pressurizing an ingredient gas mixture with a suitable means, normally a gas compressor, to a certain pressure; then leading the mixture into adsorption towers that contain an adsorbent through valves; and repeating, for example, the following cycle:
(1) A supply gas is fed through a supply inlet, and part of the product gas is fed through an outlet to equalize the pressure.
(2) The supply gas continues to be fed to pressurize further.
(3) Part of the product gas is fed into another tower, and the pressure is equalized.
(4) Through parallel current depressurization, part of the product gas is used to purge another tower.
(5) With counter current depressurization the desorption of adsorbed components is carried out.
(6) The purge is received from an outlet.
TABLE 1. ______________________________________ Time from the begin- ning of each cycle Adsorption towers (sec.) No. 1 No. 2 No. 3 ______________________________________ 0-15 Repressuriza- Counter current Pressure tion depressuriza- equalization tion (I) 15-35 Repressuriza- Discharge Parallel current tion depressuriza- tion 35-40 Repressuriza- Repressuriza- Pressure tion tion equalization (II) 40-55 Pressure Repressuriza- Counter current equalization tion depressuriza- (I) tion 55-75 Parrallel Repressuriza- Discharge current depres- tion surization 75-80 Pressure Repressuriza- Repressuriza- equalization tion tion (II) 80-95 Counter current Pressure Repressuriza- depressuriza- equalization tion tion (I) 95-115 Discharge Parrallel Repressuriza- current depres- tion surization 115-120 Repressuriza- Pressure Repressuriza- tion equalization tion (II) ______________________________________
FIG. 4 shows, as an example, an air separator utilizing an adsorption method that is used to separate the air into oxygen and nitrogen. According to this, after dust is removed from the air using an air filter 26, the air is pressurized to about 5 Kg/cm.sup.2 by an air compressor 27c, and the heat of compression is removed by a water cooler 28. Water is then removed by a water separator 29, and the air is led into adsorption towers 19a-19d.
The adsorption towers 19a-19d contain adsorbent 20a-20d, respectively, that is made of zeolite or the like having a filtering function. Of the air fed through valves 24a-24d, only the product gas component passes through the adsorbent 20a-20d and is led into a product gas holder 21 through valves 24e-24h.
Since a large amount of the air, compared with the adsorptivity of the adsorbent 20a-20d, is fed, it is necessary to desorb by depressurization in order to recover adsorptivity, followed by discharge of the waste air through valves 24i-24l.
Thus, to supply the product gas continuously, it is necessary to carry out adsorption under high pressure and desorption under low pressure periodically, and this requires at least two adsorption towers. Here, an example with 4 towers is shown. Also, 24m-24q in FIG. 4 are pressure equalization valves. The valves 24a-24q are controlled by a controller 22 to ensure smooth adsorption and desorption.
In order to make the air separator of FIG. 4 more economical, a cost reduction in equipment and in operation has to be achieved through a size reduction and an increase in operational efficiency.
To reduce equipment costs a reduction in time per cycle would be effective, and to achieve a higher efficiency a reduction in required power during pressurization and depressurization processes would be desirable.
However, according to the prior art, the pressure modulation required for the operational cycle of the adsorbent 20a-20d is achieved by pressurization up to a certain pressure and then by opening and closing of a plurality of valves 24a-24l at the inlets and outlets of the adsorption towers 19a-19d. At the beginning of an adsorption process pressure energy does not do any work, and after the completion of an adsorption process the adsorbed gas expands and its compression energy is released and lost. Thus, a large amount of power is inevitably required, and the period per cycle is commonly 1-5 min. It is easy to slow the process, but it is difficult to shorten the period per cycle due to problems associated with the durability and operational speed of the valves 24a-24l. A further reduction of the period incurs enormous costs and has been virtually impossible.